Underground drilling involves drilling a bore through a formation deep in the Earth using a drill bit connected to a drill string. During rotary drilling, the drill bit is typically rotated by a top drive or other rotary drive means at the surface, where a quill and/or other mechanical means connects and transfers torque between the rotary drive mechanism and the drill string. During drilling, the drill bit is rotated by a drilling motor mounted in the drill string proximate the drill bit, and the drill string may or may not also be rotated by the rotary drive mechanism.
Drilling operations can be conducted on a vertical, horizontal, or directional basis. Vertical drilling typically refers to drilling in which the trajectory of the drill string is vertical, i.e., inclined at less than about 10° relative to vertical. Horizontal drilling typically refers to drilling in which the drill string trajectory is inclined horizontally, i.e., about 90° from vertical. Directional drilling typically refers to drilling in which the trajectory of the drill string is inclined directionally, between about 10° and about 90°. Correction runs generally refer to wells that are intended to be vertical but have deviated unintentionally and must be steered or directionally drilled back to vertical.
Various systems and techniques can be used to perform vertical, directional, and horizontal drilling. For example, steerable systems use a drilling motor with a bent housing incorporated into the bottom-hole assembly (BHA) of the drill string. A steerable system can be operated in a sliding mode in which the drill string is not rotated and the drill bit is rotated exclusively by the drilling motor. The bent housing steers the drill bit in the desired direction as the drill string slides through the bore, thereby effectuating directional drilling. Alternatively, the steerable system can be operated in a rotating mode in which the drill string is rotated while the drilling motor is running.
Rotary steerable tools can also be used to perform directional drilling. One particular type of rotary steerable tool can include pads or arms located on the drill string near the drill bit and extending or retracting at some fixed orientation during some or all of the revolutions of the drill string. Contact between the arms and the surface of the wellbore exerts a lateral force on the drill string near the drill bit, which pushes or points the drill bit in the desired direction of drilling.
Directional drilling can also be accomplished using rotary steerable motors which include a drilling motor that forms part of the BHA, as well as some type of steering device, such as the extendable and retractable arms discussed above. In contrast to steerable systems, rotary steerable motors permit directional drilling to be conducted while the drill string is rotating. As the drill string rotates, frictional forces are reduced and more bit weight is typically available for drilling. Hence, a rotary steerable motor can usually achieve a higher rate of penetration during directional drilling relative to a steerable system or a rotary steerable tool, since the combined torque and power of the drill string rotation and the downhole motor are applied to the bit.
Directional drilling requires real-time knowledge of the angular orientation of a fixed reference point on the circumference of the drill string in relation to a reference point on the wellbore. The reference point is typically magnetic north in a vertical well, or the high side of the bore in an inclined well. This orientation of the fixed reference point is typically referred to as toolface. For example, drilling with a steerable motor requires knowledge of the toolface so that the pads can be extended and retracted when the drill string is in a particular angular position, so as to urge the drill bit in the desired direction.
When based on a reference point corresponding to magnetic north, toolface is commonly referred to as magnetic toolface (MTF). When based on a reference point corresponding to the high side of the bore, toolface is commonly referred to as gravity tool face (GTF). GTF is usually determined based on measurements of the transverse components of the local gravitational field, i.e., the components of the local gravitational field perpendicular to the axis of the drill string. These components are typically acquired using an accelerometer and/or other sensing device included with the BHA. MTF is usually determined based on measurements of the transverse components of the Earth's local magnetic field, which are typically acquired using a magnetometer and/or other sensing device included with the BHA.
Obtaining, monitoring, and adjusting the drilling direction conventionally requires that the human operator must manually scribe a line or somehow otherwise mark the drill string at the surface to monitor its orientation relative to the downhole tool orientation. That is, although the GTF or MTF can be determined at certain time intervals, the top drive or rotary table orientation is not known automatically. Consequently, the relationship between toolface and the quill position can only be estimated by the human operator, or by using specialized drilling equipment such as that described in co-pending application Ser. No. 12/234,584, filed Sep. 19, 2008, to Nabors Global Holdings, Ltd. It is known that this relationship is substantially affected by reactive torque acting on the drill string and bit.
It is understood in the art that directional drilling and/or horizontal drilling is not an exact science, and there are a number of factors that will cause a well to be drilled on or off course. The performances of the BHA are affected by downhole formations, the weight being applied to the bit (WOB), drilling fluid pump rates, and various other factors. Directional and/or horizontal wells are also affected by the engineering, as well as the execution of the well plan. At the end of the drilling process there is not presently much attention paid to, much less an effective method of, evaluating the performance of the driller at the controls of the drilling rig. Consequently, there has been a long-felt need to more accurately evaluate a driller's ability to keep the toolface in the correct orientation, and to be able to more accurately evaluate a driller's ability to keep the well on target, such as at the correct inclination and azimuth.